This invention relates generally to transportation of electrical power to sub-sea electrical equipment such as a motor driving a compressor/pump located far away from the shore, and more particularly to a system and method for continuing to power sub-sea electrical equipment during an electrical cable fault.
Transportation of electrical power to oil and gas sub-sea electrical equipment often requires high power to be transported over long distances. Transmission to sub-sea equipment is used to supply the power from an onshore utility to a point where the power is distributed among individual loads. Generally, a step down transformer is implemented in order to bring the high voltage level of the transmission stage to a lower voltage level for a distribution stage to individual units of the electrical equipment. Distribution distances are typically shorter than the transmission distance; and the voltage levels to be supplied to individual loads or load clusters are lower than the voltage levels of the transmission stage. Typically, power on the order of 50 megawatts is transmitted by high voltage alternating current (AC) transmission cables to a high voltage transformer, thereafter stepping down the voltage for a medium voltage AC distribution system.
One commonly used nominal voltage is 132 kV (which is considered to be a high voltage for power transmission). Transmission voltages of +/100 kV or higher are used in HVDC transmission projects where high power is transmitted over long distance (e.g., in transmitting 100 MW or 200 MW over a distance of 100 or 200 km).
AC transmission provides technical challenges for applications where bulk power is transmitted over long cables. The cable stray capacitance causes charging current to flow along the length of the AC cable. Because the cable must carry this current as well as the useful load current, this physical limitation reduces the current carrying capability of the cable. Because capacitance is distributed along the entire length of the cable, longer lengths produce higher capacitances, thus resulting in higher charging currents.
Typically, multiphase booster pumps require electrically driven motors delivering a shaft power between 2 MW and 6 MW. Future offshore oil and gas resource installations will require pump installations at distances above 50 km from the shore. Such distances require a high voltage power transmission; however, high voltage AC transmission is very costly when supplying single sub-sea pumps or clusters of a few sub-sea pumps only, where the power to be transmitted is at or below 20 MW.
Further, sub-sea motors driving a gas compressor typically have a higher nominal power (e.g., in the order of 10 or 15 MW). As such, sub-sea compression clusters may be required to transmit a total power in the order of 50 to 100 MW over a distance of 100 or 200 km. The transmission of high power over a distance of more than 100 km and distributing the power sub-sea is very challenging with AC transmission and distribution systems because of the high charging currents and the high number of components involved in the distribution system.
In general, DC transmission can be achieved more efficiently over long distances than AC transmission. High voltage (HV) DC transmission typically requires the usage of power electronic converters in the transmission systems that are capable of converting between HVAC and HVDC. Each switch of the converter for conventional HVDC converter topologies is designed to handle high voltages. The converter nominal voltage may range from tens-of-kilovolts to hundreds-of-kilovolts, depending upon the application. Such switches are typically configured utilizing a plurality of series connected semiconductor devices (e.g., such as insulated gate bipolar transistors (IGBTs) and thyristors). Because of the size and the high number of components involved, conventional HVDC terminals are not well suited for sub-sea installations.
Converters are also required on the load side of a power distribution system when supplying variable speed motors in addition to the power conversion required for HVDC transmission. Typically, a high voltage transformer is used to step down the voltage from the AC or DC transmission level to the voltage level used in the AC power distribution system. On the load side of the distribution system, the converters convert the power from fixed frequency AC voltage (stepped down from the transmission system) to a variable frequency AC voltage of individual motors that must be controllable over a wide speed range when driving sub-sea pumps or compressors.
Modular stacked DC converter architectures are well suited for sub-sea applications requiring transmission and distribution over long distances. Unlike other DC transmission options, e.g. where the dc transmission (link) voltage is controlled, i.e. maintained nearly constant, the dc transmission (link) current is controlled in a modular stacked dc converter. One MSDC architecture 10 is depicted in FIG. 1. The MSDC architecture gets its name from the fact that the architecture uses several dc-dc converter modules stacked and connected in series, both at the sending end and at the receiving end of the transmission link such as depicted in FIG. 1.
The sending end/top-side converters 12 comprise a set of ac-dc converters 14, which draw power from the ac mains or grid 16. Each of these converters 14 is cascaded by a dc-dc converter 18. These dc-dc converters 18 are connected in series and they are controlled so as to regulate the current in the dc cable 20 connecting the top-side 12 to the sub-sea installation 22. The receiving-end/sub-sea side 22 also comprises several dc-dc converters 19 connected in series. Each of these converters 19 is cascaded by a dc-ac inverter/motor drive 24. These dc-dc converters 19 are controlled to regulate the dc link voltage to that required by the down-stream motor drive 24. Although FIG. 1 depicts two-level converters used for the ac-dc, dc-dc and dc-ac converter modules, it shall be understood that at high power levels, multi-level stacks will be used for these converter modules.
High voltage direct current (HVDC) transmission has technical and commercial advantages that increase with the distance of the power transmission. Sub-sea power transmission is always based on sub-sea high voltage (HV) cables and umbilicals. With increasing cable length, the probability of a cable fault increases. Repairing sub-sea cables is costly and typically takes a long time i.e. months rather than weeks. In view of the foregoing, there is a need to provide an HVDC transmission system that can be kept operating regardless of a transmission cable fault.